Process for the manufacture of sulfur hexafluoride

ABSTRACT

Elemental sulfur is introduced into a cell containing an electrolyte of the composition KF.nHF (wherein n 0.9 to 3), and electrolysis is carried out with use of a carbon anode to produce sulfur hexafluoride.

United States Patent [72] Inventors l-liroshl Ukihashl Tokyo; YoshioOda, Yokohama; Manabu Suhara,

I Yokohama, all of Japan [21 Appl. No. 50,892

[22] Filed June 29, 1970 [45] Patented Nov. 30, 1971 [73] Assignee AsahlGlass Co., Ltd.

Tokyo, Japan [32] Priority July 3, 1969 [33] Japan [54] PROCESS FOR THEMANUFACTURE OF SULFUR [50] Field of Search 204/60, 61. 59. I01

[56] References Cited UNITED STATES PATENTS l ,653,605 12/1927 Ashcroft204/61 2,717,235 9/l955 Prober 204/59 2,937,123 5/l960 Muetterties204/59 3,146, I 79 8/1964 Davies 204/60 3,345,277 l0/l967 Ashleyetal.

Primary Examiner.|ohn H. Mack Assistant E.\'aminer--D. R. ValentineAu0rneyKelman and Berman ABSTRACT: Elemental sulfur is introduced into acell containing an electrolyte of the composition KF'nHF (wherein n=O.9to 3 and electrolysis is carried out with use of a carbon anode toproduce sulfur hexafluoride.

PROCESS FOR THE MANUFACTURE OF SULFUR HEXAFLUORIDE BACKGROUND OF THEINVENTION This invention relates to a process for the production ofsulfur hexafluoride. More particularly, the invention relates to aprocess for producing sulfur hexafluoride by electrolysis.

DESCRIPTION OF THE PRIOR ART It is well known to use sulfur hexafluorideas a gaseous insulating medium for sealed electrical apparatus such astransformers.

In the hitherto-known methods for the manufacture of sulfurhexafluoride, sulfur, sulfur dichloride, sulfur monochloride, carbondisulfide or hydrogen sulfide is introduced into anhydrous hydrogenfluoride and nickel anodes are used to effect the necessary electrolysis(See the specifications ofU.S. Pat. Nos. 3,345,277 and 2,717,235).

Anhydrous hydrogen fluoride has a boiling point of l9 C. and the knownmethods require low-temperature operation at 30 C. and C.

Furthermore, because of the poor electric conductivity of anhydroushydrogen fluoride, it is necessary to employ a conductive additive, forexample, potassium fluoride. The potassium fluoride causes considerabledissolution of the nickel anode and an increase in current densityaccelerates this dissolution, causing a substantial loss of nickel.

BRIEFSUMMARY OF THE INVENTION It is an object of this invention toprovide an electrolytic process for producing pure sulfur hexafluoridein commercial quantities from elemental sulfur and a KF-nHF electrolyte.

Another object is to provide an electrolytic process for producingsulfur hexafluoride in a high yield without a substantial loss of theanode.

Other objects and advantages of the present invention will becomeapparent to those skilled in the art from the following description anddisclosure.

DETAILED DESCRIPTION OF THE INVENTION, INCLUDING PREFERRED EMBODIMENTSIt has been found that electrolysis of an electrolyte of KF-nHF (whereinn equals 0.9 to 3.0) in the presence of elemental sulfur and with acarbon anode yields commercial quantities of sulfur hexafluoriderefrigeration equipment and substantially without dissolution of theanode.

In the present invention, a mixture of potassium fluoride and hydrogenfluoride in a mole ratio of HFzKF of 0.9:] to 3: I does not liberatesubstantial amounts of hydrogen fluoride vapor near its melting pointand has such a high electric con ductivity that the voltage necessaryfor electrolysis may be comparatively low.

A carbon anode is substantially immune to dissolution in the presence ofpotassium fluoride.

If the HF:KF ratio is less than 0.9:1, the vapor pressure of hydrogenfluoride over the mixture will be too high and the melting point of themixture will also be too high for a commercially practical process.Exceeding the limit of 3:! will also cause a significant increase in thevapor pressure of hydrogen fluoride over the mixture and loss ofhydrogen fluoride.

An electrolyte wherein the ratio of HF to KF is 1.8 to 2.2 has aparticularly favorable melting point and vapor pressure.

To carry the invention into practice, elemental sulfur and theelectrolyte are introduced into an electrolytic cell. The electrolyte isfirst fed into the cell, and is followed by elemental sulfur added tothe anolyte. Hydrogen fluoride gas is fed to the electrolyte to maintainthe original composition.

Elemental sulfur may be used in the solid form, such as colloidalsulfur, powdery sulfur or granular sulfur, or in the molten state.Elemental sulfur is substantially insoluble in the KF'nHF melt. Theelectrolyte temperature is selected between the melting point of themixture and the melting point plus 50 C.

If the mixture is heated to an excessively high temperature, hydrogenfluoride is evaporated. A temperature of C. to I30 C. is preferred whena mixture of 1.8 KF to 2.2 HF is to be melted.

At a voltage of 5 to I5 volts and a current density of 0.5 to 25a./dm.", sulfur hexafluoride is produced at a current eff ciency ofabout 40 to 92 percent.

At 6 to 12 volts and a current density of 4 to 12 a./dm. based on theeffective anode surface, the process of this invention gives sulfurhexafluoride continuously at a current efficiency of 65 to 92 percent.

At a voltage of less than 5 volts and a current density below 0.5a./dm.', substantially no sulfur hexafluoride will be produced. Theoutput of lower fluorinated sulfur compounds is increased and a largeranode area will have to be provided. At a voltage above 15 volts and acurrent density beyond 25 a./dm. there may be formed an insulatingfluorinated carbon layer on the surface of the carbon anode and make theelectrolysis substantially impossible.

A small amount, for example, I to 3 weight percent of the KF-nHFmixture, of lithium fluoride, aluminum fluoride, nickel fluoride, orsodium fluoride, is effective in preventing the coating of the anode atnot more than 25 a./dm.

The present invention will be further described in the followingexamples which are illustrative and by no means limitative thereof. Inthose examples, two different electrolytic cells were employed.

Type I Electrolytic Cell This electrolytic cell consists of acylindrical vessel made of iron, 240 mm. in diameter and 550 mm. indepth, and an iron cover. The vessel serves as a cathode. In the centerof the cover, there is provided an anode of amorphous carbon, I00 mm. indiameter, 480 mm. in length and I2 dm. in effective surface area. Thecover is also provided, about the carbon anode, with a cylindrical skirtof iron, 160 mm. in diameter, I50 mm. in length and 2 mm. in thickness,for separating the gases evolved from both electrodes.

The cover is further provided with holes through which elemental sulfuris fed into the anode chamber.

Type II The electrolytic cell consists of a cylindrical vessel of iron,I00 mm. in diameter and I50 mm. in depth, and a cell cover of iron. Thevessel serves as a cathode. In the center of the cover, there isprovided an anode of amorphous carbon, 40 mm. in diameter, I00 mm. inlength and LI drn. in effective surface area. The cover is alsoprovided, about the carbon anode, with a cylindrical skirt of iron forseparating the gas evolved from the electrodes. The skirt measures 70mm. in diameter, 60 mm. in length and 1 mm. in thickness. The cover isfurther provided with a pipe for admitting elemental sulfur to the anodechamber. The cell is provided, at the bottom, with a pipe for admittingan inert gas and for thereby sweeping the sulfur hexafluoride from thecell.

Acid potassium fluoride (KHF and anhydrous fluoride gas were admittedinto the cell to prepare an electrolyte. The electrolyte was held at aconstant temperature (about C. and in order to remove water from theelectrolyte, electrolysis was carried out for 24 hours at a low currentdensity (OJ-0.3 a./dm. After this preliminary electrolysis, elementalsulfur was admitted into the anode chamber, an inert gas was introduced,and the product of electrolysis was withdrawn thereby as a gas from theelectrolytic cell. The gas contained a small amount of hydrogen fluorideand was passed through a tube packed with sodium fluoride pellets.

It was washed free of byproducts, SO F SOF etc., with water and, then,with aqueous alkali solutions. The traces of S F S-,F,,,O and othercompounds occurring in this gas were thermally decomposed and, finally,the gas was passed through an alumina-filled tube to remove any residualtrace impurities.

The final gas was sulfur hexafluoride, which was analyzed by gaschromatography and by infrared and mass spectroscopic methods.

Control Example 1n the cell of type ll, the carbon anode was replacedwith a Ni plate anode, and electrolysis was carried out in the followingmanner. To the anode chamber containing 1.6 kg. of KF-ZHF melt, 15 g.colloidal sulfur was added. With the electrolyte being maintained at atemperature of 90 C., electrolysis was conducted at a current density of8.14 a./dm. The cell voltage was 5.9 volts. The gas evolved from theanode chamber contained such compounds as SF 6, SOP- SO F S F etc.

After 3 hours of electrolysis, the nickel anode was removed from thecell and examined for weight loss. It was dissolved at a rate of 3.8g./l ampere-hours. Electrolysis was further continued, whereupon theformation of a sludge in the cell bottom was observed.

EXAMPLE 1 (CELL TYPE I) lnto the electrolytic cell, 41.50 kg. of anelectrolyte of KF-Z- ISHF containing 1.1 weight percent of LiF wasadmitted, and 100 g. of colloidal sulfur was introduced into the anodechamber. Electrolysis was carried out at an electrolyte temperature of98C. and a current density of 5.0 a./dm.". The cell voltage was 8.93volts. After 3 hours of electrolysis, the gas evolved in the anodechamber was analyzed. The result was: SF 93%; SO F 1%; SOF +SF 1%; S F1%; S F D 0.5%; C0 1%; F 1%; CF 0.5%; air, 0.5%. On furtherelectrolysis, the amounts of SOF SO F CO and S F O became traces. Afterpurification, the SP was 99 percent pure, containing 0.5CF and 0.5% air.The current efficiency of sulfur hexafluoride formation was 92 percent.The conversion of hydrogen fluoride to sulfur hexafluoride was in excessof 95 percent.

EXAMPLE 2 (CELL TYPE ll) lnto the electrolytic cell, 1.6 kg. of anelectrolyte of KF'ZHF was admitted, and g. colloidal sulfur wasintroduced into the anode chamber. With the introduction of nitrogen gasat a rate of 2lN ml./min., electrolysis was conducted at an electrolytetemperature of 98 C. and a current density of 8.9 a./dm. The initialcell voltage was 7.60 volts. The concentration of sulfur hexafluoride inthe gas evolved from the anode chamber was 48 percent. The balance ofthe gas was predominantly nitrogen and, to a minor part, fluorine. Thecurrent efficiency of sulfur hexafluoride formation was 85 percent.During 3 hours of electrolysis, the carbon anode suffered substantiallyno dissolution.

EXAMPLE 3 (CELL TYPE 11) lnto the electrolytic cell, 1.6 kg. of anelectrolyte of KF'ZHF was admitted, and 15 g. colloidal sulfur wasintroduced into the anode chamber. With the introduction of nitrogen gasat a rate of 21N ml./min., electrolysis was carried out at anelectrolyte temperature of 108 C. and a current density of 4.1 a./dm.*.The initial cell voltage was 6.75 volts and 6.64 volts after 4 hours.The current efflciency of sulfur hexafluoride formation was 68 percent.

EXAMPLE 4 (CELL TYPE 11) Into the electrolytic cell, 1.6 kg. of anelectrolyte of KF'2HF was admitted, and 15 g. sulfur was introduced intothe anode chamber. With the introduction of nitrogen gas at a rate of14.5N ml./min., electrolysis was conducted at an electrolyte temperatureof 108 C. and a current density of 0.5 a./dm.. The cell voltage was 5.53volts at the start of electrolysis, while it was 5.54 after 5 hours. Thecurrent efficiency of sulfur hexafluoride formation was 45 percent.

EXAMPLE 5 (CELL TYPE ll) lnto the electrolytic cell, 1.6 kg. of anelectrolyte of KF'LBHF was admitted, and 15 g. sulfur was introducedinto the anode chamber. With the introduction of nitrogen gas at a rateof 20N ml./min., electrola'sis was conducted at an electrolytetemperature of and a current density of 1.4

a./dm. The cell voltage was 5.85 volts at the start of electrolysis, and5.81 volts after 5 hours. The current efficiency of sulfur hexafluorideformation increased with time and reached 63 percent in 5 hours.

We claim:

1. In a process of manufacturing sulfur hexafluoride by electrolysisbetween an anode and a cathode of an electrolyte essentially consistingof potassium and hydrogen fluoride and containing elemental sulfur inthe anolyte, and by recovery of the hexafluoride formed from theelectrolyte, the improvement which comprises:

a. the mole ratio of HF to KF in said electrolyte being 0.9:1

to 3:1 and b. the effective surface of the anode essentially consistingof carbon.

2. In a process as set forth in claim 1, said carbon being amorphouscarbon.

3. In a process as set forth in claim 1, the temperature of saidelectrolyte being above the melting point thereof by not more than 50 C.and the anode current density being between 0.5 2and 25 a./dm..

4. In a process as set forth in claim 3, the voltage between said anodeand said cathode being 5 to 15 volts.

5. In a process as set forth in claim 4, said current density being 4 to12 a./dm. and said voltage being 6 to 12 volts.

6. In a process as set forth in claim 1, said mole ratio being 1.8:1 to2.221.

7. [n a process as set forth in claim 6, the temperature of saidelectrolyte being 80 C. to C., the voltage between said anode and saidcathode being 5 to 15 volts, and the anode current density 0.5 to 25a./dm.

8. In a process as set forth in claim 7, said voltage being 6 to 12volts, and said current density 4 to 12 a./dm.

2. In a process as set forth in claim 1, said carbon being amorphouscarbon.
 3. In a process as set forth in claim 1, the temperature of saidelectrolyte being above the melting point thereof by not more than 50*C. and the anode current density being between 0.5 2and 25 a./dm.2. 4.In a process as set forth in claim 3, the voltage between said anode andsaid cathode being 5 to 15 volts.
 5. In a process as set forth in claim4, said current density being 4 to 12 a./dm.2, and said voltage being 6to 12 volts.
 6. In a process as set forth in claim 1, said mole ratiobeing 1.8:1 to 2.2:1.
 7. In a process as set forth in claim 6, thetemperature of said electrolyte being 80* C. to 130* C., the voltagebetween said anode and said cathode being 5 to 15 volts, and the anodecurrent density 0.5 to 25 a./dm.2.
 8. In a process as set forth in claim7, said voltage being 6 to 12 volts, and said current density 4 to 12a./dm.2.